1. Field of the Invention
The present invention relates to the field of enclosures and, more particularly, to electronic enclosures.
2. Background of the Related Art
The use of a plastic enclosure to house electronic components and assemblies is well known in the art. Most electronic enclosures, such as enclosures for notebook computers, are manufactured by an injection molding or compression molding technique. The plastic molding forms the outer shell or xe2x80x9cskinxe2x80x9d which provides the necessary structural rigidity, but is light in weight.
Several key requirements are specified when the enclosure is utilized for notebook computers. As noted above, the enclosure should be rigid, but light in weight, so that it can be hand-carried. The enclosure (or casing) should be resistant to cracking or breaking. For example, the enclosure should not shatter, if dropped. Further, the enclosure should be thermally conductive to dissipate heat, so that heat generated by internal components, such as power supplies, can be adequately transferred to the outer surface. Finally, some form of EMI/RFI shielding is needed to electrically shield the internal electronics.
One technique in practice utilizes an absolac/polycarbonate (ABS/PC) resin mix to fabricate the enclosures. Typically, a 60/40 mix of ABS/PC is employed in injection or compression molding to fabricate enclosures with thickness in the range of 1.5 to 2.0 millimeters (mm). A minimum thickness of about 1.5 mm is necessary for this thermoplastic material to provide adequate structural rigidity. The ABS/PC mix offers strength, impact resistance and is economically, cost effective.
However, several disadvantages are noted with the ABS/PC material. For example, the thermal conductivity of ABS/PC resin is quite low (typically less than 0.1 Watts per meter-Kelvin (W/m-K)), so that the heat spreading (dissipation) capability of the plastic is poor. Accordingly, many of today""s notebook computers have xe2x80x9chot spotsxe2x80x9d along the external casing. Additionally, the ABS/PC plastic has poor electrical conductivity so that the interior surface of the enclosure requires some form of metallization (whether a metal skin or sprayed coating) for EMI/RFI shielding.
One technique to improve the properties of the ABS/PC resin material is to introduce graphite fibers into the resin. Graphite fibers are uniformly distributed in the resin when the enclosure is fabricated. Since graphite has higher thermal conductivity than ordinary ABS/PC, the graphite laden ABS/PC improves the thermal dissipation of the plastic. However, when significant amounts of graphite fibers are introduced to improve the thermal properties of the plastic, the amount of graphite present causes the graphite/ABS/PC resin based plastic to become brittle. This causes the impact resistance of the enclosure to degrade and increases the chances that the enclosure will shatter when dropped.
Accordingly, it would be advantageous to provide an enclosure having enhanced thermal conductivity, but without suffering the degradation of impact resistance. The present invention provides for such a scheme in which thermal conductivity is enhanced for a plastic enclosure, but in which the enclosure is not susceptible to breakage from impact, such as when the enclosure is dropped.
The present invention describes an improved electronic enclosure and the fabrication of such an enclosure. The electronic enclosure of the preferred embodiment is formed from a molded plastic, comprised of graphite fibers dispersed in an absolac/polycarbonate (ABS/PC) resin mix. The graphite loading in the formed enclosure is directional (non-homogeneous), so that the concentration of the graphite varies across the thickness of the formed plastic. The graphite concentration is highest along the interior surface of the enclosure and lowest (or none) along its outer surface. In the preferred graphite/ABS/PC enclosure, the graphite concentration proximal to the interior surface has a value of around 50% and a value of zero along the outer surface.
By having a high graphite concentration loading along the interior surface, thermal conductivity is improved to dissipate the heat away from the internal components. In many instances, hot spots along the outer surface are removed or reduced, due to the high thermal conductivity of the graphite along the interior surface. The improved thermal conductivity allows heat to conduct horizontally along the enclosure, as well as toward the exterior of the enclosure. Although high concentrations of graphite would cause the plastic to become brittle and the case to break (shatter), the enclosure of the present invention does not suffer from degraded impact performance. Since the high concentration levels of graphite are only along the interior surface, the structural rigidity of the ABS/PC based plastic is still retained for most of the thickness of the formed enclosure. Furthermore, the high concentration layer of graphite along the interior surface provides enhanced EMI/RFI shielding, so that a separate metallic skin along the interior of the enclosure is not necessary.